1. Field of the Invention
The invention is generally related to the production of hydrocarbons from subsea formations and more particularly to the support of riser pipe used in such production.
2. General Background
In the offshore drilling and production industry, a pipe is often used to carry product (oil or natural gas) from the offshore production site to a collection and storage facility which may be a tanker or an on shore facility. For offshore structures that rest on the sea floor, such as a jacket or compliant tower, the flexing effect of environmental conditions such as waves and currents have a minimal effect on the design considerations when connecting the pipe to the offshore structure. However, for floating offshore structures the design of the interface between the pipe and offshore structure must take into account the bending motions, and resulting fatigue and stresses, that the pipe and interface must endure during the life of the structure. The steel catenary pipeline riser approach is generally considered as the most cost-effective means for transporting products to and from floating offshore production vessels.
For a floating structure such as a TLP (tension leg platform) or a semi-submersible, the typical configuration of this riser is for the pipe to be suspended from the side of the floating vessel from a support platform that is located just below the water surface (fifty to one hundred feet).
For a floating structure such as a spar vessel, the pipe for the riser may enter the interior are of the spar vessel at the keel or along the side of the spar vessel at a selected depth.
A disadvantage of previous catenary riser support configurations for floating structures is that the configuration normally has been limited to a certain riser diameter and narrow range of departure angles from the floating structure.
The invention addresses the above disadvantage. What is provided is a universal catenary riser support that can be designed to accommodate all riser pipe diameters typically considered for production of offshore hydrocarbons and allows the catenary riser to exit from the vessel at any azimuth angle and at a wide range of angles from the vertical. The support structure at the keel of the offshore structure is provided with a receptacle to receive a curved riser segment. The curved riser segment is adapted to be received in the receptacle. The curved riser is also adapted to receive a vertical riser section through the offshore structure. Relative motions between the catenary riser and the offshore structure are accommodated by a tapered section of riser or flexible joint attached to the curved riser section. A removable plug may be provided in the curved riser section to prevent water from entering the catenary riser during installation in the offshore structure.